We have developed an in vitro system to induce immunoglobulin secretion by malignant B cells (lymphomas and leukemias). The immunoglonbulin secreted by these cells (IgM) is purified on an affinity column developed in our laboratory. This highly purified monoclonal immunoglobulin is used for the immunization of mice and development of monoclonal (hybridoma) antibodies specific to idiotypic determinants associated with the malignant cells. Antibodies have been produced and are being applied to investigations of differentiation and malignant transformation of B-cell neoplasms. Where appropriate, they will be administered to patients for imaging and passive immunotherapy. Preliminary evidence in another institution has shown that such antibodies may be efficacious in certain types of low-grade B-cell lymphomas.